The present invention relates to copolymers of ethylene and vinyl esters of tertiary carboxylic acids and their use as flow improvers for mineral oils.
Crude oils and middle distillates such as gas oil, diesel oil or heating oil, obtained by distillation of crude oils contain, depending on the origin of the crude oils, different amounts of n-paraffins, which crystallize out as lamellar crystals when the temperature is lowered and in some cases agglomerate with inclusion of oil. This results in a deterioration in the flow properties of these oils or distillates, giving rise to problems, for example in the recovery, transport, storage and/or use of the mineral oils and mineral oil distillates. In the case of mineral oils, this crystallization phenomenon can lead to deposits on the pipe walls during transport through pipelines, especially in the winter, and in individual cases, for example when the pipeline is shut down, even to complete blockage thereof. The precipitation of paraffin can also cause difficulties in storage and further processing of the mineral oils. Thus, it may be necessary in winter to store the mineral oils in heated tanks. In the case of mineral oil distillates, blockage of the filters in diesel engines and furnaces may occur owing to the crystallization, with the result that reliable metering of the fuels is prevented and complete interruption of the fuel or heating medium feed may occur.
In addition to the traditional methods for eliminating the paraffins which have crystallized out (thermally, mechanically or by means of solvents), which relate only to the removal of the precipitates already formed, recent years have seen the development of chemical additives (so-called flow improvers or paraffin inhibitors) which physically interact with the precipitating paraffin crystals and thus modify their shape, size and adhesion properties. The additives act as additional crystal seeds and partially crystallize out with the paraffins, resulting in a larger number of smaller paraffin crystals with modified crystal shapes. A part of the action of the additives is also explained by dispersing of the paraffin crystals. Modified paraffin crystals have less tendency to agglomerate, so that the oils into which these additives have been introduced can be pumped or processed even at temperatures which are often more than 20xc2x0 lower than in the case of oils not containing additives.
The flow and low-temperature behavior of mineral oils and mineral oil distillates is described by stating the pour point (determined according to ISO 3016) and the cold filter plugging points (CFPP; determined according to EN 116). Both characteristics are measured in xc2x0 C.
Typical flow improvers for crude oils and middle oil distillates are copolymers of ethylene with carboxylic esters of vinyl alcohol. Thus, according to DE-A-11 47 799, oil-soluble copolymers of ethylene and vinyl acetate having a molecular weight between about 1,000 and 3,000 are added to mineral oil distillate, or combustion fuels having a boiling point between about 120 and 400xc2x0 C. Copolymers which contain from about 60 to 99% by weight of ethylene and from about 1 to 40% by weight of vinyl acetate are preferred. They are particularly effective if they were prepared by free radical polymerization in an inert solvent at temperatures of from about 70 to 130xc2x0 C. and pressures of from 35 to 2,100 atm (gage pressure) (DE-A-19 14 756).
Other polymers used as flow improvers contain, in addition to ethylene and vinyl acetate, for example 1-hexene (cf. EP-A-0 184 083), diisobutylene (cf. EP-A-0 203 554) or an isoolefin of the formula 
in which R and Rxe2x80x2 are identical or different and are hydrogen or C1-C4-alkyl radicals (EP-A-0 099 646). Copolymers of ethylene, alkenecarboxylic esters and/or vinyl esters and vinyl ketone are also used as pour point depressants and for improving the flow behavior of crude oils and middle distillates of crude oils (EP-A-0 111 888).
In addition, copolymers based on xcex1,xcex2-unsaturated compounds and maleic anhydride are also used as flow improvers. DE-196 45603 describes copolymers of from 60 to 99 mol % of structural units derived from ethylene and from 1 to 40 mol % of structural units which are derived from maleic acid, its anhydride or its imides. The disadvantage of using such compounds is the complicated preparation process, namely high-pressure mass copolymerization of ethylene and maleic anhydride and subsequent imidation of the anhydride groups.
DE-1 162 630 discloses copolymers of ethylene and vinyl esters of straight-chain fatty acids having 4 to 18 carbon atoms as pour point-depressing additive for mineral oil distillates. Neocarboxylic acids are not mentioned.
EP-A-0 217 602 discloses ethylene copolymers with vinyl esters carrying C1- to C18-alkyl radicals as flow improvers for mineral oil distillates having boiling ranges (90-20%) of less than 100xc2x0 C.
EP-A-0 648 256 discloses, inter alia, copolymers of ethylene and vinyl esters of C1-C28-carboxylic acids, but once again no neocarboxylic acids.
EP-A-0 493 769 discloses terpolymers which are prepared from ethylene, vinyl acetate and vinyl neononanoate or neodecanoate and their use as additives for mineral oil distillates.
EP-A-0 584 651 discloses copolymers of ethylene with a tertiary, saturated carboxylic acid for the production of films which have a melt flow index MFI (190/2,16) of from 0.1 to 50 g/10 min. Owing to their high molecular weight and their low solubility, such copolymers are however unsuitable for use as mineral oil additives.
EP-A-0 746 598 discloses copolymers of ethylene and dialkyl fumarates as a mixture with mineral oils which have a cloud point below xe2x88x9210xc2x0 C.
The efficiency of the known additives for improving the properties of mineral oil fractions is dependent, inter alia, on the origin of the mineral oil from which they were obtained and hence in particular on its composition. Additives which are very suitable for establishing specific properties of fractions of a crude oil can therefore lead to completely unsatisfactory results in distillates of crude oils of another origin.
Additives which have a wide range of use, i.e. which substantially improve the flow properties of mineral oils and mineral oil fractions of different origins at lower temperatures, are now available. Nevertheless, there are cases where they prove to be not very useful or completely useless, for example because they make little contribution to increasing the low-temperature flowability. The reasons for this are varied; the exploitation of raw materials not used to date, the changed processing of the primary products and new market requirements may be mentioned as examples.
In particular, diesel fuels which are distinguished by lower environmental pollution during combustion and are characterized by a very low sulfur content of less than 500 ppm and in particular less than 100 ppm and a low aromatics content can be treated with conventional flow improvers only to an inadequate extent, if at all. Problems are encountered in particular with the winter grades of diesel fuels produced for use under arctic conditions and having extreme low-temperature properties, such as, for example, a cloud point of less than xe2x88x9210xc2x0 C. and in particular less than xe2x88x9215xc2x0 C., very narrow distillation cuts having boiling ranges of 20 to 90% by volume less than 100xc2x0 C., in particular less than 80xc2x0 C. and in some cases also less than 60xc2x0 C., and a final boiling point limited to about 360xc2x0 C. and in particular to about 320xc2x0 C. The low-temperature properties of such distillates can at present be satisfactorily improved only by adding low-boiling, low-paraffin components, such as, for example, kerosene.
It was therefore the object to develop novel additives for improving the flowability of such mineral oil grades or mineral oil fractions in which the additives of the prior art have only an unsatisfactory action. Furthermore, it was the object to find additives which can be prepared in a process which is less complicated than, for example, that in the case of the imide derivatives of maleic acid as comonomers.
The present invention achieves this object by providing flow improvers for mineral oils, which contain copolymers of ethylene with the vinyl esters of tertiary carboxylic acids, also referred to below as neocarboxylic acids.
The invention relates to copolymers of low olefins and vinyl esters, containing
a) from 85 to 97 mol % of bivalent structural units of the formula 1
xe2x80x94CH2xe2x80x94CR1R2xe2x80x94xe2x80x83xe2x80x831
xe2x80x83in which R1 and R2, independently of one another, are hydrogen or methyl, and
b) at least 3 mol % of bivalent structural units of the formula 2 
in which R3 is saturated, branched C6-C16-alkyl which has a tertiary carbon atom, wherein R3 is bonded by its tertiary carbon atom to the carboxyl function.
R1 and R2 are preferably hydrogen. R3 is preferably a neoalkyl radical having 7 to 11 carbon atoms, in particular a neoalkyl radical having 8, 9 or 10 carbon atoms. The neoalkanoic acid from which the abovementioned neoalkyl radicals can be derived are described by the formula 3: 
Rxe2x80x2 and Rxe2x80x3 are alkyl radicals having together preferably 5 to 9, in particular 6, 7 or 8, carbon atoms. The vinyl ester used for the copolymerization accordingly has the formula 4: 
The copolymer according to the invention preferably comprises not more than 15, in particular from 5 to 10, mol % of structural units of the formula 2.
The copolymers according to the invention can be prepared by the conventional copolymerization methods, such as, for example, suspension polymerization, solution polymerization, gas-phase polymerization or high-pressure mass polymerization. High-pressure mass polymerization at pressures of, preferably, from 50 to 400, in particular from 100 to 300, MPa and temperatures of, preferably, from 50 to 300xc2x0 C., in particular from 100 to 250xc2x0 C., is preferred. The reaction of the monomers is initiated by initiators forming free radicals (free radical chain initiators). This class of substances includes, for example, oxygen, hydroperoxides, peroxides and azo compounds, such as cumyl hydroperoxide, tert-butyl hydroperoxide, dilauroyl peroxide, dibenzoyl peroxide, bis(2-ethylhexyl) peroxydicarbonate, tert-butyl perpivalate, tert-butyl permaleate, tert-butyl perbenzoate, dicumyl peroxide, tert-butyl cumyl peroxide, di(tert-butyl) peroxide, 2,2xe2x80x2-azobis(2-methylpropanonitrile) and 2,2xe2x80x2-azobis(2-methylbutyronitrile). The initiators are used individually or as a mixture comprising two or more substances in amounts of from 0.001 to 20% by weight, preferably from 0.01 to 10% by weight, based on the monomer mixture.
Preferably, the copolymers according to the invention have melt viscosities at 140xc2x0 C. or from 20 to 10,000 MPas, in particular from 30 to 5000 MPas, especially from 50 to 2000 MPas.
For a given composition of the monomer mixture, the desired melt viscosity of the copolymers is established by varying the reaction parameters pressure and temperature and, if required, by adding moderators. Hydrogen, saturated or unsaturated hydrocarbons, e.g. propane, aldehydes, e.g. propionaldehyde, n-butyraldehyde or isobutyraldehyde, ketones, e.g. acetone, methyl ethyl ketone, methyl isobutyl ketone or cyclohexanone, or alcohols, e.g. butanol, have proven useful as moderators. Depending on the intended viscosity, the moderators are used in amounts of up to 20% by weight, preferably from 0.05 to 10% by weight, based on the monomer mixture.
The comonomers suitable for the preparation of copolymers according to the invention are in particular vinyl neooctanoate, neononanoate, neodecanoate, neoundecanoate and neododecanoate. These esters can be prepared, for example, by vinylation of the neocarboxylic acids obtainable from olefins, CO and H2O by the Koch carboxylic acid synthesis (Rxc3x6mpp: Chemie-Lexikon, Thieme-Verlag, 9th edition, pages 4881 and 4901).
The copolymers according to the invention may contain up to 4% by weight of vinyl acetate or up to 5 mol % of further comonomers. Such comonomers may be, for example, vinyl esters, vinyl ethers, alkyl acrylates, alkyl methacrylates or higher olefins having at least 5 carbon atoms. Preferred higher olefins are hexene, 4-methylpentene, octene and diisobutylene.
In order to obtain copolymers of the claimed composition, monomer mixtures which, in addition to ethylene and, if required, a moderator, contain from 1 to 50% by weight, preferably from 3 to 40% by weight, of vinyl esters are used.
The different polymerization rates of the monomers are taken into account by virtue of the fact that the composition of the monomer mixture differs from the composition of the copolymer. The polymers are obtained as colorless melts which solidify to waxy solids at room temperature.
The high-pressure mass polymerization is carried out batchwise or continuously in known high-pressure reactors, for example autoclaves or tube reactors; tube reactors have proven particularly useful. Solvents, such as aliphatic and/or aromatic hydrocarbons or hydrocarbon mixtures, benzene or toluene, may be present in the reaction mixture. The solvent-free procedure is preferred. In a preferred embodiment of the polymerization, the mixture comprising the monomers, the initiator and, if used, the moderator is fed to a tube reactor via the reactor inlet and via one or more side branches. The monomer streams may have different compositions here (EP A-0 271 738).
The copolymers according to the invention are added to mineral oils or mineral oil distillates in the form of solutions or dispersions. These solutions or dispersions contain, preferably, from 1 to 90, in particular from 10 to 80, % by weight of the copolymers according to the invention. Suitable solvents or dispersants are aliphatic and/or aromatic hydrocarbons or hydrocarbon mixtures, for example gasoline fractions, kerosene, decane, pentadecane, toluene, xylene, ethylbenzene or commercial solvent mixtures, such as Solvent Naphtha, (copyright)Shellsoll AB, (copyright)Solvesso 150, (copyright)Solvesso 200, (copyright)Exxsol, (copyright)ISOPAR- and (copyright)Shellsol D grades. Mineral oils or mineral oil distillates improved in their rheological properties by the novel polymeric compounds contain from 0.001 to 2, preferably from 0.005 to 0.5, % by weight of copolymer, based on the distillate.
The copolymers according to the invention can be used as flow improvers also in the form of mixtures which comprise polymers of the claimed type but different qualitative and/or quantitative composition and/or different viscosity (measured at 140xc2x0 C.). The mixing ratio (in parts by weight) of the copolymers can be varied over a wide range and may be, for example, from 20:1 to 1:20, preferably from 10:1 to 1:10. In this way, the flow improvers can be specifically adapted to individual requirements.
For optimizing the efficiency as a flow improver for specific substrates, the copolymers according to the invention can also be used together with one or more oil-soluble coadditives which by themselves improve the cold flow properties of crude oils, lubricating oils or fuel oils, with the same result. Examples of such coadditives are polar compounds which disperse paraffin (paraffin dispersants), and comb-like polymers.
Thus, the novel copolymers can be used as a mixture with paraffin dispersants. These additives reduce the size of the paraffin crystals and prevent the paraffin particles from settling out but ensure that they remain dispersed in colloidal form with substantially reduced tendency to settle out. Oil-soluble polar compounds having ionic or polar groups, e.g. amine salts and/or amides, which are obtained by reacting aliphatic or aromatic amines, preferably long-chain aliphatic amines, with aliphatic or aromatic mono-, di-, tri- or tetracarboxylic acids or anhydrides thereof have proven useful as paraffin dispersants (cf. U.S. Pat. No. 4,211,534). Other paraffin dispersants are copolymers of maleic anhydride and xcex1,xcex2-unsaturated compounds which, if required, may be reacted with primary monoalkylamines and/or aliphatic alcohols (cf. EP-A-0 154 177), the reaction products of alkenylspirobislactones with amines (cf. EP-A-0 413 279) and, according to EP-A-0 606 055, reaction products of terpolymers based on xcex1,xcex2-unsaturated dicarboxylic anhydrides, xcex1,xcex93-unsaturated compounds and polyoxyalkenyl ethers of lower unsaturated alcohols.
Finally, in a further embodiment of the invention, the novel copolymers are used together with comb-like polymers as flow improvers. These are understood as being polymers in which hydrocarbon radicals having at least 8, in particular at least 10, carbon atoms are bonded to a polymer skeleton. Preferably, they are homopolymers whose alkyl side chains contain at least 8 and in particular at least 10 carbon atoms. In the case of copolymers, at least 20%, preferably at least 30%, of the monomers have side chains (cf. Comb-like Polymersxe2x80x94Structure and Properties; N. A. Platxc3xa9 and V. P. Shibaev, J. Polym. Sci. Macromolecular Revs. 1974, 8, 117 ff). Examples of suitable comb-like polymers are fumarate/vinyl acetate copolymers (cf. EP-A 0 153 176), copolymers of a C6-C24-xcex1-olefin and an Nxe2x80x94C6- to C22-alkylmaleimide (cf. EP-A-0 320 766), and furthermore esterified olefin/maleic anhydride copolymers, polymers and copolymers of xcex1-olefins and esterified copolymers of styrene and maleic anhydride.
The mixing ratio (in parts by weight) of the copolymers according to the invention with paraffin dispersants or comb-like polymers is in each case from 1:10 to 20:1, preferably from 1:1 to 10:1.
The novel copolymers and their mixtures with one another and with coadditives can be used alone or together with other additives, for example with dewaxing assistants, corrosion inhibitors, antioxidants, lubricity additives or sludge inhibitors. They reduce CFPP of oils with extreme low-temperature properties in which conventional additives have no action or increase the CFPP.